1. Field of the Invention
This disclosure relates to determining material properties, and more particularly to a system and method for determining thickness and composition of layers.
2. Description of the Related Art
Characteristic analysis of layers within measurement samples is desirable for many applications. For example, the amount and type of adhesive applied to a tape product or the amount and type of metal or paint applied to a device for corrosion protection is critical for functionality of the products. Often, the most important characteristics of the layer include thickness, composition and uniformity. The method and system as described herein are discussed primarily with respect to the analysis of thin films within semiconductor devices. The term xe2x80x9cthin filmxe2x80x9d is commonly used within the semiconductor industry when referring to layers deposited upon a semiconductor wafer during the fabrication of a transistor.
As production volumes and efforts to improve process control increase in the integrated circuit fabrication industry, the ability to accurately characterize semiconductor processes and the materials associated with such processes in a timely manner becomes more critical. Inaccurate analysis of one or more process parameters within the processing of a semiconductor wafer may hinder or prohibit the function of a transistor, leading to a reduction in production efficiency and transistor quality. The characterization of thin films is especially important, since the effectiveness and reliability of thin films play an important, central role in the operation of a transistor. Therefore, thin films must be accurately analyzed in order to meet a transistor""s functionality requirements. Unfortunately, many current analysis techniques employ complex, expensive systems that do not coincide with the desire to increase production efficiency and improve process control within the semiconductor industry.
In order for a thin film to be effective, it must conform to strict electrical, chemical, and structural requirements. Specialized materials are selected for thin films to perform specific functions of the transistor. These materials may include, but are not limited to, metallic, semiconducting, and dielectric materials or a combination of them. Often thin films are doped with impurities to heighten the effectiveness of the material used. Thus, the composition of the material must be closely monitored to insure the correct material or combination of materials is applied, along with insuring the correct amount of material is applied. Accordingly, the composition, thickness and uniformity of a thin film all play crucial roles in the function of a transistor.
At present, it is difficult to find an analytical technique suitable for use in a semiconductor fabrication area that can characterize in a simple, accurate and cost-effective manner the thickness, uniformity and composition of a thin film. Many current techniques require expensive and large pieces of equipment that are not used within a fabrication area due to size and cleanliness requirements. Several of these techniques actually destroy the sample being measured, thus increasing the manufacturing cost and time requirements. It is nonetheless useful to discuss some of the analytical tools used currently in the semiconductor industry, for these tools do point out some difficulties and shortcomings associated with characterizing thin films.
One of the analytical methods in current use is Secondary Ion Mass Spectroscopy (hereinafter referred to as SIMS). In SIMS, a sample to be studied is bombarded with a primary beam of energetic ions. These ions sputter away ionized particles, or secondary ions, from the surface of the sample. The secondary ions are directed into a mass spectrometer, which identifies the ions as a function of their mass to charge ratio. Continued sputtering dislodges particles and secondary ions located below the surface of the sample. Thus, SIMS has the ability to analyze elements embedded within the sample as a function of sample depth. Therefore, SIMS can be used to measure the amount of material embedded within a thin film.
Although SIMS depth resolution, lateral resolution, and sensitivity continue to improve year after year, several drawbacks are inherent with SIMS measurements. The biggest drawback is the fact that SIMS is a destructive technique. SIMS sputters away layer after layer of material from the surface of the sample; thus, it is not feasible to use SIMS as a bench-top process control station, which could monitor the amount of material embedded within a thin film. Also, SIMS is a very bulky, complex, expensive method requiring complicated, maintenance-intensive machinery. For instance, SIMS instruments typically occupy an entire room in a mid-sized laboratory and consist of several vacuum pumps, valves, powerful magnets, energy filters, ion sources, and complex data analysis tools.
Another technique, which may be used in the semiconductor industry, is Auger electron spectroscopy (hereinafter referred to as AES). In AES, an energetic, primary electron beam is directed at the surface of a sample. The primary electron beam interacts with atoms at and near the surface of the sample, dislodging electrons from energy shells of the sample. As an energy shell is vacated, an electron within a higher energy state may fill the vacant position. The electron filling the once-vacant state releases energy characteristic of the transition in energy levels. This energy then interacts with the atom and ejects an electron of a lower energy state. Such an ejected electron is termed an Auger electron and has energy characteristic of the process, which caused its ejection. Because an ejected Auger electron has an energy characteristic of the energy levels of the atom from which it is ejected, one may determine the composition of the sample being studied by measuring the Auger electrons. Because Auger electrons cannot escape from great depths within the bulk of a sample, AES is considered a surface-sensitive analysis technique. It is commonly used to study materials present at a depth within fifty Angstroms from the sample""s surface.
In order to study the composition of a sample deeper below the surface, it is necessary to sputter away atoms from the surface of the sample being studied. Thus, to measure the quantity of materials embedded within a thin film deeper than approximately fifty Angstroms, ion sputtering must often be used. Although providing excellent lateral resolution and possessing the ability to probe very small areas, AES suffers from the same major drawback as does SIMSxe2x80x94when probing beneath the surface of a thin film, sputtering is required which effectively destroys the sample. Also, like SIMS, AES requires expensive, complex machinery, which may become maintenance intensive. A typical AES system consists of vacuum pumps (AES is most effective when carried out at pressures of approximately 10xe2x88x9210 torr and lower) and an ion beam for sputtering the sample.
Another technique, which may be utilized in microelectronics characterization, is X-ray Photoelectron Spectroscopy (hereinafter referred to as XPS). In this technique, an x-ray beam is directed at a sample, and the interaction of x-ray photons with the atoms of the sample causes the ejection of electrons from the sample. The kinetic energy of the ejected electrons is characteristic of the sample being studied. Like AES, only electrons from the top 1-10 monolayers are emitted from the sample. Thus, XPS is similarly a surface-sensitive technique. Like AES, if XPS is to probe within the thin film, destructive sputtering must be employed. Also, similar to AES, XPS systems are quite complex, expensive, and may become maintenance intensive. A typical system consists of powerful vacuum pumps, an electrostatic energy analyzer, and a complicated data analysis system.
X-ray Emission Spectroscopy (hereinafter referred to as XES) is yet another technique in use in the semiconductor field for the purpose of analyzing thin film parameters. In XES, an electron beam impinges upon a sample, creating electron vacancies. When these vacancies are filled, characteristic x-ray photons may be emitted and correlated with the elemental composition and thickness of the sample being studied. Although a very powerful technique, one major drawback to XES is that x-ray photons as deep as five microns below the surface are sometimes emitted and detected by XES systems, creating a lot of background noise. Furthermore, XES is not necessarily able to distinguish atoms embedded within a thin film from atoms embedded within a silicon substrate. Consequently, XES may not be a reliable method to measure thin films containing multiple elements. For example, XES may not be able to detect the amount of silicon embedded within a thin film of titanium directly above a silicon substrate. In such a case, XES would detect silicon from within the thin titanium film as well as from the silicon substrate.
Another possible technique is X-ray Fluorescence (hereinafter referred to as XRF). In XRF techniques, a beam of primary x-ray photons is directed at the surface of a sample. Atoms within the sample absorb the x-rays and emit secondary x-ray photons with characteristic energy levels. The elemental compositions of materials on and under the surface of the wafer may then be determined from the measured energy levels (or wavelengths) of the emitted secondary x-rays. One drawback to XRF is the presence of background radiation, which limits the sensitivity of the device. Primary x-ray photons may lose energy when scattered by atoms of the target material. Such scattered primary x-ray photons which reach the x-ray detector of an XRF instrument create an unwanted background intensity level which secondary x-ray photons must exceed in order to be discerned. Thus, the smallest amount of an element that may be detected in a sample using an XRF instrument is largely determined by the background intensity level at the energy level (or corresponding wavelength) associated with characteristic secondary xrays emitted by that element.
X-ray Reflectivity is yet another method which may be used for the thickness and compositional analysis of thin films on semiconductor surfaces. Primary x-ray photons are reflected off the surface of a targeted material at an angle that is representative of the surface composition and thickness. Unfortunately, this method is only effective for analysis of the surface film, since the reflectance angle of the x-ray is only representative of the material from which it is reflected. Therefore, surface etch techniques must be employed in an effort to determine thickness and compositional characteristics of layers beneath the upper most layer. These techniques are very time consuming and are destructive in nature, requiring expendable samples. Such tests cannot be routinely performed economically or efficiently.
Another commonly used technique to determine thin film thickness is Scanning Electron Microscopy Cross-Sectional Analysis (hereinafter referred to as SEM). SEM is used on a sample that is sliced at the junction of analysis. A primary electron beam, typically the same electron beam used in conjunction with X-ray Emission Spectroscopy, is then directed at a cross section of a sample, while a photograph is simultaneously taken. Both the photograph and electron beam are used to determine the depths of the exposed layers. As with other techniques described above, this system is inefficient in that it is destructive. Therefore, it is not conducive to a manufacturing environment, in which production efficiency is a high priority.
And still other technique used is called Rutherford Backscattering Spectroscopy (hereinafter referred to as RBS). RBS uses high energy helium ions to bombard the surface and subsurface of a sample. Some ions are backscattered with characteristic energy loss and distance to that of the composition of the thin film. However, there are several disadvantages to this method. Often, the technique is unable to distinguish one element from another and the sensitivity to elements with light atomic weight is poor. Furthermore, the method is unable to measure samples of great depths. Therefore, in order to measure layers far beneath the surface of the sample, the sample must be etched and thus the sample is destroyed. The ion accelerator required is also impractically large, complex, and expensive for use in a semiconductor production environment.
Lastly, one method that allows characterization of thin films within a fabrication area is Picosecond Ultrasonic Laser Sonar (hereinafter referred to as PULSE Technology(trademark)). PULSE Technology(trademark) utilizes laser-induced ultrasounds that result in temperature gradients, which are measured by sonar. However, PULSE Technology(trademark) is dependent on several parameters including, for example, density, surface roughness and temperature. As with many systems that are dependent on several parameters, interpretation of results is more complex and typically not as reliable as a system with fewer variables. Therefore, a system employing PULSE Technology(trademark) is often not used due to its complexity and variability in measurements.
It would therefore be advantageous to create a system and a method in which to address the aforementioned disadvantages of thin film analysis techniques. It would further be beneficial to create a system and method that is worthy of being placed within a fabrication area in addition to analyzing the uniformity of the thin film thickness across a surface. The system and method should be suitable for analysis of layers, including elements of both small and large atomic weights.
The problems outlined above may be in large part addressed by a system and a method for determining the thickness and elemental composition of one or more layers within a sample. The technique and apparatus offer an easy and inexpensive manner in which to analyze the sample. Furthermore, such an analysis is designed to be performed within a semiconductor fabrication environment of the layering application, allowing immediate analysis of the sample. The resulting simplification of the manufacturing process may improve production efficiency and process control capabilities.
The system and method as described herein impinges an incident x-ray beam on an exposed surface of a measurement sample. The sample may consist of a semiconductor wafer or other device with one or more layers. In the application of semiconductor wafers, the layers are typically thin films, which have been applied to a semiconductor substrate. The term, thin film, may refer to the layers applied to a wafer. There may be multiple thin films horizontally and vertically adjacent from each other within a measurement sample. Furthermore, the layers may be deposited or thermally grown. The materials for the thin films are selected for their chemical and physical properties and therefore, the composition and thickness of the thin film plays a vital role in the function of a transistor.
In the system and method as described herein, an incident x-ray beam is directed at an angle relative to an exposed surface of a sample by an x-ray source. The angle may be selected from a wide range of degrees, specifically a range greater than zero degrees and less than ninety degrees. Although the incident x-ray beam may be applied at any angle within the previously prescribed range, shallow angles (i.e. less than five degrees) are often used since they typically produce a larger angle of refraction upon penetration through a sample as compared to an incident x-ray beam applied at a large angle relative to the sample. The larger angle of refraction may allow for a more precise measurement of a parameter relating to the size of the angle of refraction. In addition to the impinging angle, an incident x-ray beam may be directed at a preset intensity. In one embodiment, the preset intensity is a single, constant value. However, another embodiment exposes the sample to a multiple of preset intensities, wherein the preset intensities are either presented individually for a given amount of time or in a continuous manner. Corresponding angles of impingement may also differ as preset intensities change. It is postulated that a plurality of intensities and angles may offer additional information regarding the characteristics of a thin film as compared to an individual preset intensity and angle.
Continuing with the system and method as described here, a portion of the incident x-ray beam passes through the sample and refracts through each layer to produce a transmitted x-ray beam. The angle of refraction through the sample relates to the angle between the transmitted x-ray beam and a line perpendicular to the exposed surface of the sample. The transmitted x-ray beam is then collected by a detector, which is positioned to face the surface of the sample from which the transmitted x-ray beam is emitted. Preferably, this surface is opposite from the exposed surface of the sample on which the incident x-ray beam may be impinged. The detector may comprise a lithium-drifted silicon detector or a sodium iodide silicon detector, but other suitable x-ray detector materials may be used. The main purpose of the detector is to detect the transmitted x-ray beam, so that the intensity and the angle of refraction of the transmitted x-ray beam may be measured. In one embodiment, the detector is adapted to measure the intensity and angle of refraction of the transmitted x-ray beam.
The thickness of the sample may then be determined by comparing the intensity collected by the detector to calibration data associated with a sample of similar thickness to that of the measurement sample, since the intensity of the transmitted x-ray beam is a function of the thickness of the layers within the measurement sample. In another embodiment, the angle of refraction may be compared to calibration data associated with a sample of similar composition to that of the sample being measured. Consequently, the angle of refraction is a function of the elemental composition of the layers. Alternatively, the system and method as described here may be adapted to compare both the collected intensity and angle of refraction of the transmitted x-ray beam to calibration data in order to determine the thickness and elemental composition of the measurement sample, respectively.
The calibration data is prepared prior to the measuring the sample and includes at least one pre-measured intensity value and/or at least one pre-measured angle of refraction corresponding to an x-ray beam transmitted through a comparison layer of known thickness and elemental composition. This pre-measured intensity and pre-measured angle of refraction are used as a reference for measurements of samples prepared with similar characteristics as the composite. The elemental composition and thickness characteristics of the known composite may be determined by one or more of the aforementioned techniques as described in the Background of the Invention. The method and system described herein may be used to obtain an intensity and angle of refraction of a transmitted x-ray beam emitted from the composite. The results may then be correlated to the corresponding film characteristics previously measured.
One of the more important advantages of the method and system as described herein is that many types of samples, including those with multiple elements and thicknesses, may be measured as long as a calibration standard is prepared in relation to the characteristics of the sample. This includes elements of both low and high atomic numbers. Moreover, the individual layers within samples may comprise one or more elements. In this embodiment, the entire composition of sample is measured and compared to calibration data corresponding to a known composite containing the same number of layers with equivalent compositions and thicknesses.
Alternatively, the system and method described herein may also comprise a computer readable storage medium including program instructions for simplifying the measurement of sample characteristics in a variety of manners. For example, the coordinate positions of the sample, incident x-ray beam and detector may be preprogrammed into the computer in order to acquire the desired positions in a fast and efficient manner. These positions are often predetermined from the establishment of the calibration data. Additionally, the comparison of the angles of refraction and intensity levels of the calibration data and measurement sample may be computed in a statistical process control (SPC) software program. The pre-measured intensity and pre-measured angle of refraction of the known composite relating to the calibration data may be used as target values. Preferably, a warning signal or alarm is activated if either the thickness or angle of refraction of the measurement sample is not within the allowed process parameters, which are preprogrammed into the SPC program.
Another embodiment of the system as described herein includes a stage on which to place the measurement sample. The stage is typically a platform configured for the size of the sample, however stages of a variety of shapes and sizes may be used. The stage may further include one or more openings wherein the section of the sample that emits the transmitted x-ray beam is not in contact with the stage. Therefore, the transmitted x-ray beam may pass through the opening to the detector without incurring additional refraction through the stage. It should be noted that the stage may have openings or may not have openings, but the pre-measured intensity and pre-measured angle of refraction values of the calibration data corresponding to a known composite should be measured with the same stage to account for the refraction of the x-ray beam through the stage.
Lastly, another embodiment of the method and system recited herein would allow for multiple measurements to be taken on one sample or a plurality of samples. This would increase the efficiency and production throughput of samples. Furthermore, a stage may be configured to hold multiple samples to further increase the productivity of the system and method. The positions of the incident x-ray beam, stage, and detector would vary upon the measurement of different locations and samples.
As noted above, the measurement sample typically used in the method and system as described herein is a semiconductor wafer. The advantages of this method and system include an easy and inexpensive manner in which to determine the thickness and composition of layers or thin films within the measurement sample. Furthermore, the system may be used within a semiconductor wafer fabrication area. It may additionally be included in the process sequence of the fabrication process. This benefit enables the process of semiconductor fabrication to be more efficient while improving reliability of the transistor through process control applications.